1. Field of the Invention:
This invention relates to a semiconductor laser device and a method for the production of the laser device. More particularly, it relates to a semiconductor laser device with a small amount of ineffective current and a method for the production of the laser device.
2. Description of the Prior Art:
Semiconductor laser devices are widely used as a light source for optical disks and optical communication systems they will be used, for example, as a light source for laser printers and optical wiring. Quantum-well laser devices having a quantum well structure in the active region have a possible high electrical-optical conversion ratio (that is, a low threshold current) and a possible high frequency modulation, and it is expected that they will be used as optical devices in the future.
The main structures of the current-injection region and the light-confining region of such quantum-well lasers that are formed into a striped shape include the planar stripe structure, the buried heterostructure for which liquid phase epitaxy (LPE) is used, and the ridge guide structure.
However, each of these structure has the following disadvantages: With the planar stripe structure, the amount of ineffective current that flows outside of oscillation region is large. With the buried heterostructure, it is difficult to control the thickness of the buring layer, and the production yield is extremely poor. With the ridged guide structure, it is necessary that the control of the depth to which etching is done for the ridged shape be extremely precise. In these ways, it is difficult to build-in quantum-well lasers that have a structure with small amounts of ineffective current and satisfactory production yield by conventional methods.
To solve these problems, many attempts have been made to change the multiple quantum-well layers to an alloy layer by the use of impurity diffusion techniques or optical-anneal impurity implantation techniques, and to bring about radiation in the remaining quantum-well portion. For example, a method for the production of quantum-well layers using an Si diffusion technique has been published by R. L. Thomton in Applied Physics Letters, vol. 47, pp. 1239-1241 (1985). However, the resulting alloy layer has crystal defects which give undesirable effects on the lift span of the laser devices obtained.
Hayakawa et al. have developed and disclosed a method by which ridged guide structures can be made with satisfactory precision in U.S. Pat. No. 4,567,060. FIG. 11 shows a laser device that is produced by this method.
This laser device is produced as follows: First of all, on substrate 501, an n-Al.sub.x Ga.sub.1-x As cladding layer 502, an Al.sub.y Ga.sub.1-y As active layer 503, a p-Al.sub.x Ga.sub.1-x As first cladding layer 504, a p-GaAs etching-stop layer 505, a p-Al.sub.x Ga.sub.1-x As second cladding layer 506, and a p-GaAs contact layer 507 (x&gt;y) are successively grown by either molecular beam epitaxy (MBE) or metal organic chemical vapor deposition (MO-CVD). The thickness of the etching-stop layer 505 is extremely thin, 200 .ANG. or less. Thereafter, ridge 520 is formed by an etching technique. Because there is this etching stop layer 505 between the p-first cladding layer 504 and the p-second cladding layer 506, it is possible to control this etching so that it reaches only to the p-first cladding layer 504. Therefore, the thickness of the cladding layer outside of the ridge 520, which can give rise to ineffective current, depends upon the thickness d.sub.3f of the p-first cladding layer 504. Moreover, because the etching-stop layer 505 is only 200 .ANG., there is little superposition of light thereon, and it is possible to prevent the optical loss based on light absorbance at the etching-stop layer 505.
However, the inventors of this invention investigated further, and found that even when the thickness of the GaAs etching-stop layer 505 was set to be 200 .ANG. or less, so long as light is absorbed by this etching-stop layer, the oscillation threshold current is 3-10 times that when there is no etching stop layer. Therefore, in this new kind of structure as well, improvement is still needed in view of the oscillation threshold current.